When trying to mold a horse into an English frame the biggest thing you need is forward impulsion. Naturally, a horse will have forward energy, whether they are under saddle or loose playing. We need to channel that energy into a controlled, methodical energy with drive from behind and elevation in the front.
When starting this process with a young horse the first thing is to create good cues by clucking and/or tapping with a crop. As your horse accelerates from your cues, reward him, and eventually he will move forward faster and with more acceleration. The biggest thing to remember here is to move your hands forward
every time you ask for impulsion and "open the door". Then slowly ask for a headset as the horse moves forward.
Next, the horse needs to willingly accept pressure against the bars of his mouth, along with being free in the neck. If your horse is stiff necked you can do different "bitting" exercises to "free" him up. If he resents the pressure on the bars of his mouth you can try wrapping the bit with latex or using elastic side reins to get him used to the pressure.
If you have forward impulsion, a soft mouth, and a free neck your on your way. Remember to reward your horse and he will try even harder for you the next time. Most importantly, it takes
a lot of time to make an "English" horse, and a lot of muscle for him to hold the frame.